A diol or glycol's a chemical compound containing two hydroxyl groups (-OH groups) Vicinal diols have hydroxyl groups attached to adjacent atoms. Examples of vicinal diol compounds are ethylene glycol and propylene glycol. Geminal diols have hydroxyl groups bonded to the same atom. General properties In general, organic geminal diols readily dehydrate to form a carbonyl group. For example, carbonic acid ((HO)2C=O)'s unstable and has a tendency to convert to carbon dioxide (CO2) and water (H2O).… (More on Glycol)